Abstract
Plant phytochromes are thought to transduce light signals by mediating the degradation of phytochrome-interacting transcription factors (PIFs) through the N-terminal photosensory module, while the C-terminal module, including a histidine kinase-related domain (HKRD), does not participate in signaling. Here we show that the C-terminal module of Arabidopsis phytochrome B (PHYB) is sufficient to mediate the degradation of PIF3 specifically and to activate photosynthetic genes in the dark. The HKRD is a dimerization domain for PHYB homo and heterodimerization. A D1040V mutation, which disrupts the dimerization of HKRD and the interaction between C-terminal module and PIF3, abrogates PHYB nuclear accumulation, photobody biogenesis, and PIF3 degradation. By contrast, disrupting the interaction between PIF3 and PHYB’s N-terminal module has little effect on PIF3 degradation. Together, this study demonstrates that the dimeric form of the C-terminal module plays important signaling roles by targeting PHYB to subnuclear photobodies and interacting with PIF3 to trigger its degradation.
Highlights
Plant phytochromes are thought to transduce light signals by mediating the degradation of phytochrome-interacting transcription factors (PIFs) through the N-terminal photosensory module, while the C-terminal module, including a histidine kinase-related domain (HKRD), does not participate in signaling
When the nuclear fractions of BCY and BCY18 were compared, the nuclear BCY18 was 3- and 12-fold less than the nuclear BCY in R light and darkness, respectively (Fig. 3f, g). These results demonstrate that D1040V disrupts the function of the Cterminal module of phytochrome B (PHYB) in nuclear accumulation, photobody localization, and PIF3 degradation
The widely accepted model indicates that PIF3 degradation is triggered by the light-induced interaction with the N-terminal photosensory module of PHYB through specific residues in the knot lasso[23, 50,51,52], whereas the C-terminal output module is considered to participate in only subcellular localization and dimerization but not directly in signaling, and the HKRD is thought to be dispensable for PHYB function[53, 63]
Summary
Plant phytochromes are thought to transduce light signals by mediating the degradation of phytochrome-interacting transcription factors (PIFs) through the N-terminal photosensory module, while the C-terminal module, including a histidine kinase-related domain (HKRD), does not participate in signaling. The relative nuclear fractions of BCY18 is 3.01 and 11.99 fold less than those of BCY in R light and darkness, respectively the C-terminal module of PHYB is biologically active in mediating PHYB signaling.
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